Electricity meters typically connect to a junction between utility power lines and a load. For example, an electricity meter is often connected at the point at which the electrical system of a residence connects to the utility power line. The meter may thereby perform measurements regarding the energy consumed by residence for billing and other purposes. Similarly, electricity meters are often connected at the point at which the electrical system of a business or other non-residential consumer connects to the utility power line, again for billing and other purposes.
Different consumers employ electricity for different purposes. As result various types of electrical service are available to suit the needs of the consumer. For example, most residences employ basic consumer electrical devices and only require a single-phase electrical service. Larger facilities and/or facilities, however, can include multiphase electrical devices or high power equipment, and therefore require one of several types of polyphase or multiphase electrical service. Examples of known service types include four-wire wye, four-wire delta and three-wire wye services. These types of electrical service require different electricity meters that are suited to perform polyphase electrical energy consumption measurements.
Electricity meters, both single phase and polyphase, sometimes include disconnect switches that can be controlled to disconnect and reconnect a load to the utility power lines. Disconnect switches can be used to implement prepaid electricity services in residential meters. Disconnect switches may also be employed for load shedding in residential and larger polyphase systems. In a prepaid service, the disconnect switch automatically disconnects the load from the power lines once the customer has consumed the prepaid amount of energy. The customer may then purchase more prepaid services to have the power reconnected via the disconnect switch. In load shedding, one or more phases of a polyphase system may be disconnected to reduce consumption by a particular facility. Other implementations of electricity disconnect switches are known.
In one type of load shedding application, a meter disconnect switch arrangement is programmed to disconnect the load from the power lines when the energy consumption for one or more finite periods of time exceeds a predetermined threshold. Such a threshold is known in the art as a demand threshold. The demand threshold represents a limit of average power that can be consumed over a period of time. The period of time is a predetermined interval of between 1 and 60 minutes. If the energy consumption measurements within the meter indicate that the average power exceeds the demand threshold for the defined time period, then the disconnect switch arrangement disconnects the at least one phase of electrical power from the load. This demand threshold feature allows a customer to limit its power consumption when the power consumption begins to get too high. Such an arrangement can result in significant cost savings because energy price rates often increase as a function of consumption.
Accordingly, many meters with disconnect switch arrangements are configured to disconnect the customer load from the power line when the demand threshold is exceed for any one of an ongoing sequence of time periods. In general, meters having this feature may be programmed by external devices, such as portable computing devices, that communicate with the meter circuitry via an optical port on the meter.
An additional feature that has become increasingly important is the potential use of a disconnect switch arrangement for fusing purposes. In particular, there is an interest to use the disconnect switch to disconnect utility power if the power provided to a load is exceedingly high, thereby resulting in a possible safety concern due to potential overload. The fusing application typically operates in the same manner as the load shedding/demand limiting application except the threshold typically represents an instantaneous power/energy measurement, as opposed to an average or cumulative measurement over a demand period.
One difference between fusing applications and the load shedding/demand limiting applications discussed further above is that customers intuitive associate fusing/safety applications with current limits, as opposed to energy or power limits. Accordingly, there is a need for a disconnect switch application that can controllably disconnect power to a load based on load current, as opposed to energy demand or average power consumption.
A known method of addressing this need is to employ a measurement circuit within the meter to perform current measurements, and then compare the current measurements to a threshold. Control circuitry within the meter may then cause the disconnect switch to disconnect the load from the power lines if the current exceeds the defined current threshold.
While adding the firmware and circuits configured to perform such current analysis would provide current-based disconnect, such additional firmware and operations further complicate utility meters. Moreover, such additional firmware and operations are difficult to implement in meters that have already been installed. Accordingly, there is a need for a method to test for current limits in a meter that does not require new or different dedicated meter firmware.